Mention inheritance when defining default text processing language.

[stevenatkin]

http://w3c.github.io/bp-i18n-specdev/#sec_text_processing_lang

http://w3c.github.io/bp-i18n-specdev/#lang_inherit

I would add inheritance to the general description in the checklist and consider merging with "Content within the resource should inherit the language of the text-processing declared at the resource level, unless it is specifically overridden"

The specification should indicate how to define the default text-processing language for the resource as a whole and any inheritance models.